1. Field of the Invention
The present invention relates to a vibrator that may be incorporated in a small-sized device such as a cellular phone, a watch and other portable devices. More particularly, the present invention relates to a vibrator used in a cellular phone, e.g. to inform the user of an incoming call.
2. Description of the Related Art
Conventionally, a portable terminal device such as a cellular phone has a vibrator incorporated therein as a device that informs a user of an incoming call by vibrations of the phone body. Regarding the vibrator, there has been a demand for miniaturization. Under these circumstances, the present applicant proposed a small-sized vibrator as shown in FIG. 8 (see Japanese Patent Application Publication No. 2004-261684).
The proposed vibrator includes a voice coil 42 and a vibrating member 44 having a magnet 44a to be disposed in the coil 42 and a yoke 44b facing the magnet 44a across the coil 42. The vibrator further includes suspensions 48 and 46 that resiliently support the vibrating member 44. When the voice coil 42 is supplied with an alternating current, the vibrating member 44 is reciprocated in the axial direction of the voice coil 42, thereby generating vibrations.
The suspensions 46 and 48 comprise pairs of leaf spring members 46a, 46b and 48a, 48b respectively. Each of the leaf spring members 46a, 46b, 48a and 48b has one end thereof fixed to a casing (described below). Each of the other ends of the leaf spring members 46a, 46b, 48a and 48b is attached to the yoke 44b in order to resiliently support the vibrating member 44.
The voice coil 42, the vibrating member 44 and the suspensions 46 and 48 are housed in a casing having a frame 54 surrounding the vibrating member 44. The casing further has a first end plate 52 to which the voice coil 42 is fixed and which closes one opening of the frame 54 and a second end plate 60 closing the other opening of the frame 54. The first end plate 52 is formed as a printed wiring board having a wiring pattern connected to the voice coil 42 and terminals. Further, the casing has a spacer 56 between the frame 54 and the first end plates 52 and has another spacer 58 between the frame 54 and the second end plate 60. One end of each of the leaf spring members 46a and 46b is secured between the frame 54 and the spacer 56. One end of each of the leaf spring members 48a and 48b is secured between the frame 54 and the spacer 58.
As another example of the suspension structure that resiliently supports a vibrating member, a spiral leaf spring as shown in FIG. 6 has been developed. The spiral leaf spring 30 has an inner ring portion 31 and an outer ring portion 32 shifted in the axis direction thereof relative to the inner ring portion 31. Arcuate portions 33 are set between the inner ring portion 31 and the outer ring portion 32. One end of each arcuate portion 33 is connected to the inner ring portion 31, and the other end thereof is connected to the outer ring portion 32. A pair of such spiral leaf springs 30 is disposed at both axial ends of a vibrating member 39. The inner ring portions 31 of the two spiral leaf springs 30 are respectively fixed to two end surfaces of the vibrating member 39, and the outer ring portions 32 of the leaf springs 30 are fixed to a casing (not shown), thereby resiliently supporting the vibrating member 39.
Therefore, in this case, the spiral directions of the arcuate portions 33 of the spiral leaf spring 30 (i.e. the directions of extension of the arcuate portions 33 from their joints with the associated outer ring portions 32 to their joints with the associated inner ring portions 31) are opposite to each other as seen from one side of the axial direction of the vibrating member 39. Accordingly, when the vibrating member 39 moves in one axial direction, e.g. the direction indicated by the arrow A in FIG. 6, the arcuate portions 33 of one spiral leaf spring 30 (the lower spiral leaf spring 30 as seen in FIG. 6) behave so as to expand in a circumferential direction indicated by the arrow E in FIG. 6, whereas the arcuate portions 33 of the other spiral leaf spring 30 (the upper spiral leaf spring 30 in FIG. 6) behave so as to contract in a circumferential direction indicated by the arrow F in FIG. 6. Consequently, the vibrating member 39 slightly rotates about its own axis. When the vibrating member 39 moves axially in the opposite direction to the above, the arcuate portions 33 of the two spiral leaf springs 30 behave opposite to the above. Accordingly, the vibrating member 39 vibrates around its own axis as it reciprocates.
In the vibrator having the vibrating member 39 supported with the pair of ring-shaped spiral leaf springs 30 as stated above, the frequency at which a resonance occurs varies undesirably. FIG. 7 is a graph showing the relationship between the frequency and acceleration of such a vibrator with respect to the input voltage. As will be understood from FIG. 7, the vibrator in question suffers a change in resonance frequency in response to a change of the input effective voltage. This may be caused by the above-described behavior of the pair of leaf spring 30.
Thus, the vibrator using a pair of spiral leaf springs 30 as stated above cannot stably obtain large vibrations using resonance. Hence, it is difficult to surely inform a user of an incoming call when this vibrator is applied to a cellular phone.